Ships are surrounded by water so that where they get it from and it is converted to fresh water by the reverse osmosis procedure. The end result is probably purer than you get at home. Not quite sure how often the sheets are changed but on P&O they always appear to be clean and fresh. Towels get changed twice daily and there are separate towels for the swimming pools, which again are changed after use.

The Sheets are changed very regulary however in regard to the water the ships have two Healys which make their own fresh water from seawater.
It works in the same way as the water cycle: evaporation, condensation, precipitation, transportation. Water is pumped in from the sea (transportation). It is piped into a place where they add pressure and heat so the water boils (evaporation). The water vapor turns into water drops as it hits a cooling tray (condensation). Those drops fall into a pan (precipitation). The salt and minerals of ocean water are left behind and fresh water flows through a bromine filter to make sure it is safe to drink. Finally, it is pumped all over this huge ship through a maze of pipes (transportation).
An evaporator can make about 6,600 gallons a day. There are two of them, just in case one needs cleaning or it breaks. All the water is stored in two, huge, containers. Each one holds 14,000 gallons. Everyone on the ship should take sea showers to save water.
In a sea shower, you get yourself wet and turn off the water. Then you soap up and scrub. Last, you turn on the shower once more to rinse.
So have no worries that you wont have fresh water onboard for washing and brushing your teeth etc but again it is not advised that you drink the water (as you wouldn't in forgein countries)
Regards
Hawke.

Wow! thanks for answering my question Hawkes and Ingle that was certainly interesting ever since i booked the Cruise I have been wondering how they had so much water to shower etc and was actually wondering if the cruise ship had to pump in fresh water at every stop but now I know I was wrong so thanks a million.

Wow! thats really interesting thanks Hawkes and Ingles I actually thought they might have water pumped in every time they berthed somewhwere so thank a million for answering my question as now I know whetre the water to shower etc comes from.